feat(realtime): push channel.joined/left events to user-scoped rooms

Backend half of the plugin push-based channel sync (companion to
nav/Fabric.OpenclawPlugin#1 follow-up). Before this, the OpenClaw
fabric inbound had to poll `/api/channels?guildId=...` every 60s to
discover newly-joined channels (any DM another user just dragged the
agent into). Now the server tells the agent's socket directly so
sub/unsub is realtime.

Changes:
- realtime.gateway.ts:
  * handleConnection joins the socket into a `user:<userId>` room.
    All of a user's connected sockets now share that room.
  * New `emitToUser(userId, event, data)` helper that emits into
    that room. No-op for offline users (next connect resyncs via the
    plugin's initial channel-list fetch).
- channels.service.ts:
  * Inject RealtimeGateway (RealtimeModule is @Global, no module
    plumbing needed).
  * Private `notifyMembership(kind, channelId, userIds, extra)`
    helper that emits `channel.<kind>` (joined|left) with payload
    {channelId, userId, xType, occurredAt}.
  * create(): emit channel.joined to every seeded member (creator +
    explicit memberUserIds + triage on-duty).
  * joinChannel(): emit channel.joined to userId (only if the row was
    actually inserted, idempotent on existing membership).
  * leaveChannel(): emit channel.left to userId iff a row was
    actually deleted.

Event shape:
  {
    channelId: string,
    userId: string,
    xType?: string,
    occurredAt: ISO string,
  }

Client-side contract (fabric plugin):
  socket.on('channel.joined', m => socket.emit('join_channel', {channelId: m.channelId}))
  socket.on('channel.left',   m => socket.emit('leave_channel', {channelId: m.channelId}))

The plugin keeps its 60s polling resync as a safety net for missed
events (transient socket drops between emit and reconnect, partial
failures, etc).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/realtime/realtime.gateway.ts

@@ -96,6 +96,10 @@ export class RealtimeGateway implements OnGatewayConnection, OnGatewayDisconnect
     const userId = result.user.id || this.userIdFromClient(client);
     client.data.userId = userId;
     this.onlineUsers.add(userId);
+    // Per-user room: lets server code emit user-scoped events (e.g.
+    // channel.joined when membership changes) without bookkeeping a
+    // userId→sockets map. All of this user's sockets receive the event.
+    client.join(`user:${userId}`);
     this.server.emit('presence.online', {
       userId,
       onlineCount: this.onlineUsers.size,
@@ -171,6 +175,14 @@ export class RealtimeGateway implements OnGatewayConnection, OnGatewayDisconnect
     this.server.to(`channel:${channelId}`).emit(event, data);
   }
 
+  // Emit a user-scoped event to all sockets currently connected for `userId`
+  // (via the `user:<userId>` room joined in handleConnection). No-op for
+  // offline users — the next connect's initial channel-list fetch covers it.
+  emitToUser(userId: string, event: string, data: Record<string, unknown>): void {
+    if (!userId) return;
+    this.server.to(`user:${userId}`).emit(event, data);
+  }
+
   // Emits message.created per-recipient so each carries its own `wakeup` flag.
   async emitMessageCreated(
     channelId: string,